This invention relates to solar cells and, more particularly, to an anti-reflective coating for solar cells that are responsive to light across the entire visible light spectrum including light in the short wavelength region.
The use of silicon solar cells, which convert light energy to electrical energy is well known in both terrestrial and outer space applications. Light incident upon a silicon solar cell is absorbed by the bulk semiconductor of the cell and results in the generation of electron-hole pairs (i.e., carriers). Ideally, the carriers are spatially separated by the semiconductor junction without recombination at the junction. The carriers may be collected at opposite surfaces of the solar cell by metallic current collectors thereby creating a current flow.
The efficiency (i.e., electrical power output/power input of incident useful light) of a solar cell is directly related to the amount of useful light entering the silicon cell. The useful light for a given solar cell may be defined as electromagnetic energy at those wavelengths which, when absorbed by the solar cell, will result in the generation of carriers at the cell junction. However, the efficiency of the solar cell is limited due to reflection of useful light striking the top surface of the solar cell. To reduce this problem of light reflection, an anti-reflective coating is applied to the surface through which light enters the solar cell.
As is well known in the art, the particular environment in which a solar cell will be used will determine the specific mechanical, chemical and optical properties that its anti-reflective coating must have. In space applications, where reliability in a hostile environment and over extended time periods is required, the presence of these properties is essential to a successful mission.
As a fundamental optical property, the anti-reflective coating should reduce reflection of the useful light. In space applications, where a cover slide, usually quartz, is placed over the anti-reflective coating to shield the solar cell from harmful radiation, the index of refraction of the anti-reflective coating should be between that of the quartz cover slide and the underlying solar cell, generally in the range of 2.0-2.5. Another required optical property of an anti-reflective coating is transparency. The anti-reflective coating should not absorb any of the useful light, but should enable passage of such light to the underlying solar cell. In the final analysis, the optical properties required for an anti-reflective coating are dependent upon the refractive indices of both the underlying solar cell and the cover slide, as well as the wavelength response of that solar cell.
In a co-pending application entitled "Fine Geometry Solar Cell" by Joseph Lindmayer, Ser. No. 184,393, now U.S. Pat. No. 3,811,954, assigned to the assignee of the present invention, a novel solar cell is described for which useful light includes light across the entire visible spectrum, particularly light in the blue-violet region of the spectrum. This region corresponds to the short wavelength region of light at about 0.3-0.5 microns. Heretofore, a solar cell with this capability has not been known in the art. In order to utilize efficiently a solar cell having a wavelength response that extends into the short wavelength region, it is necessary to employ an anti-reflective coating that would not absorb light across the entire visible spectrum, i.e., 0.3-1.1 microns, and would have a refractive index between that of the coverslide and the solar cell.
The anti-reflective coating also must satisfy certain mechanical and chemical criteria. In addition to environmental and life-time considerations, these criteria would be determined by the physical characteristics of the solar cell. A solar cell responsive to light in the short wavelength region, particularly a cell constructed in accordance with the teachings of the Lindmayer application referenced above, would require an anti-reflective coating to satisfy certain specific criteria. For example, in the short wavelength responsive solar cell described in the application to Lindmayer, the n-p junction is only about 1000-2000 A from the top surface of the solar cell. Under these conditions the anti-reflective coating would damage the shallow junction if the coating penetrated into the solar cell. Also, any mechanical stress produced at the anti-reflective coating -- semi-conductor interface must be small so that such stress will not damage the junction.
In addition, the anti-reflective coating should not degrade upon exposure to ultraviolet light in a vacuum. The effect of such degradation could be a change in the index of refraction of the anti-reflective coating and the absorption of light at short wavelengths. Also, with respect to silicon solar cells, there is a phenomena known as dispersion whereby the index of refraction of the silicon becomes greater at the shorter wavelengths. Therefor, the anti-reflective coating should have a relation to wavelength that matches the variable refractive index of silicon.
Still other criteria of an anti-reflective coating relate to its stability, adhesion qualities and hardness. The anti-reflective material should be chemically stable in that it should not change composition during processing where it may be exposed to temperature, chemicals and moisture; and should not change during shelf storage in order to assure constant optical properties. The adhesion of the anti-reflective coating to the solar cell should be excellent so as to ensure that delamination would not occur during processing or exposure to moisture or temperature cycling. Finally, the anti-reflective material should be hard enough so that it would not be damaged, during manufacture or use, particularly during coverslide attachment.
An anti-reflective material which meets all the above criteria has been described in a co-pending application entitled "Tantalum Pentoxide Anti-Reflective Coating" by Joseph Lindmayer, et al, Ser. No. 249,024, now abandoned, assigned to the assignee of the present invention. The anti-reflective coating described in the application to Lindmayer, et al comprises amorphous, tantalum pentoxide (Ta.sub.2 O.sub.5). The present application describes another anti-reflective coating which meets all of the above criteria but has the advantage of easier manufacture under low temperature conditions.